Method for Functionalizating Carbon Naontubes Utilizing Peroxides

ABSTRACT

A method for functionalizing the wall of single-wall or multi-wall carbon nanotubes involves the use of acyl peroxides to generate carbon-centered free radicals. The method allows for the chemical attachment of a variety of functional groups to the wall or end cap of carbon nanotubes through covalent carbon bonds without destroying the wall or endcap structure of the nanotube. Carbon-centered radicals generated from acyl peroxides can have terminal functional groups that provide sites for further reaction with other compounds. Organic groups with terminal carboxylic acid functionality can be converted to an acyl chloride and further reacted with an amine to form an amide or with a diamine to form an amide with terminal amine. The reactive functional groups attached to the nanotubes provide improved solvent dispersibility and provide reaction sites for monomers for incorporation in polymer structures. The nanotubes can also be functionalized by generating free radicals from organic sulfoxides.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 11/585,368, filed Oct. 24, 2006, which is adivisional application of U.S. patent application Ser. No. 10/714,014(now U.S. Pat. No. 7,125,533), filed Nov. 14, 2003, which claimspriority to U.S. Provisional Patent Applications 60/426,784, filed Nov.15, 2002, and 60/483,817, filed Jun. 30, 2003. All of the priorityapplication are incorporated herein by reference in their entirety.

This invention was made with support from the Robert A. Welch Foundation(Grant No. C-0109), the Texas Higher Education Coordinating Board'sAdvanced Technology Program (Grant Nos. 003604-0026-2001), and theNational Science Foundation under grant number CHE-0450085. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to carbon nanotubes, particularly thefunctionalization of carbon nanotubes, and more particularly to a methodfor functionalizing the wall of carbon nanotubes utilizing peroxides.

BACKGROUND OF THE INVENTION

Since their discovery in 1993, single-walled carbon nanotubes (SWNT)have become an area of wide-reaching research and development activitydue to their exceptional chemical and physical properties, includinghigh strength, stiffness, and thermal and electrical conductivity. SWNTare hollow, tubular fullerene molecules consisting essentially ofsp²-hybridized carbon atoms typically arranged in hexagons andpentagons. Single-wall carbon nanotubes typically have diameters in therange of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usuallygreater than about 50 nm.

Multi-wall carbon nanotubes are nested single-wall carbon cylinders andpossess some properties similar to single-wall carbon nanotubes.However, since single-wall carbon nanotubes have fewer defects thanmulti-wall carbon nanotubes, single-wall carbon nanotubes are generallystronger and more conductive.

There is considerable interest in the chemical modification ofsingle-wall carbon nanotubes to take advantage of single-wall carbonnanotubes' remarkable tubular framework structure in variousapplications, particularly, in the engineering of multi-functionalmaterials. SWNT derivatized with organic functional groups can provide ahigh binding affinity and selectivity through formation of eitherhydrogen or covalent bonds. Through functionalization, SWNT can exhibitimproved solubility in common organic solvents, as well as improvedmaterial properties and processability of composites, including fibersand nanotube-reinforced composite materials, such as those based onorganic and inorganic polymers. SWNT that have been chemicallyderivatized with hydrophilic substituents, such as those containingterminal hydroxyl or carboxylic acid groups, are particularly attractivefor medical and biological applications.

Exohedral SWNT functionalization can generally be classified into threemain categories: 1) non-covalently bonded, supramolecular complexation,wrapping and coating with detergents and polymers, such as given inInternational Patent Publication, “Polymer-wrapped Single-wall CarbonNanotubes” WO 02/016257 published Feb. 28, 2002, incorporated herein byreference in its entirety; 2) generation and functionalization of openand closed tube ends, such as given in International Patent Publication,“Carbon Fibers formed from Single-wall Carbon Nanotubes, WO 98/39250published Sep. 11, 1998, incorporated herein by reference in itsentirety; and 3) direct chemical functionalization of the nanotubesidewalls using addition reactions, such as given in “ChemicalDerivatization of Single-Wall Carbon Nanotubes to Facilitate SolvationThereof; and Use of Derivatized Nanotubes,” WO 00/17101 published Mar.30, 2000, incorporated herein by reference in its entirety.

Of the two chemically-bonded functionalization categories, thefunctionalization referred to as “end-derivatization,” or“end-functionalized” will be defined herein to include bonds on edgesand open tube ends. “Sidewall derivatization” or “sidewallfunctionalization” will be defined herein to include bonds made to thewall that keep the carbon-carbon bonds of the wall intact. “End-capderivatization” or “end-cap functionalization” will be defined herein toinclude bonds made to the end cap that keep the carbon-carbon bonds ofthe end cap intact. Regardless of whether the single-wall carbonnanotubes are derivatized on their ends, sides, end caps, or combinationthereof, the SWNT will be referred to as derivatized SWNT forconvenience and clarity.

End-functionalization of single-wall carbon nanotubes often proceedsthrough oxidation routes to form shortened nanotubes with carboxylicacid groups at the tube ends which can be further derivatized byreactions with a chlorinating agent, such as thionyl chloride, andlong-chain amines or by esterification. Carboxylic acid functionalitycan also be created on SWNT edges and defects on partially etched(“unzipped”) side walls by oxidative treatment with various oxidants.

Sidewall functionalization of carbon nanotubes, in which the nanotubewalls are kept intact, has been much more difficult to achieve than openend-functionalization. Methods to functionalize SWNT sidewalls withorganic groups include fluorination (see E. T. Mickelson, et al., Chem.Phys. Lett. 1998, 296, 188), followed by subsequent reactions withreactions with alkyl lithium and metal alkoxides (see P. J. Boul, etal., Chem. Phys. Lett. 1999, 310, 367, R. K. Saini, et al., J. Am. Chem.Soc. 2003, 125, 3617, and E. T. Mickelson, et al., J. Phys. Chem. B,1999, 103, 4318-4322), as well as by Grignard reagents (see V. N.Khabashesku, et al., Acc. Chem. Res. 2002, 35, 1087, and Khabashesku, V.N. and Margrave, J. L. “Chemistry of Carbon Nanotubes” in TheEncyclopedia of Nanoscience and Nanotechnology, S. Nalwa, Ed. AmericanScientific Publ. 2003) or diamines (see J. L. Stevens, et al., NanoLett. 2003, 3. 331) and reactions with aryl diazonium salts (see J. L.Bahr, et. al., J. Am. Chem. Soc. 2001, 123, 6536-6542 “Bahr”),azomethine ylides (see V. Georgakilas, et al., J. Am. Chem. Soc. 2002,124, 760; V. Georgakilas, et al., J. Chem. Soc. Chem. Commun. 2002,3050; D. Pantarotto, et al., J. Am. Chem. Soc. 2003, 125, 6160);carbenes (see Y. Chen, et al., J. Mat. Res. 1998, 13, 2423-2431, J.Chen, et al., Science, 1998, 282, 95-98, and M. Holzinger, et al.,Angew. Chem. Int. Ed. 2001, 40, 4002-4005 (“Holzinger”)); nitrenes (seeHolzinger) and organic radicals (see Holzinger, H. Peng, et al., J.Chem. Soc. Chem. Commun., 2003, 362, and Y. Ying, et al., Org. Lett.2003, 9, 1471).

One method of functionalizing fullerenes with moieties having terminalcarboxylic acid groups has been demonstrated with C₆₀ using a two-stepprocess (Bingel 2+1 cycloaddition reaction followed by deesterification)yielding carboxylated methanofullerene structures. (see Kini, V. U.;Khabashesku, V. N.; Margrave, J. L. Rice Quantum Institute SixteenthAnnual Summer Research Colloquium. Aug. 9, 2002, Abstr. p. 25.) However,when applied to single-wall carbon nanotubes, the process was much lessefficient due to the inertness of single-wall carbon nanotube to carbeneaddition via Bingel-type reaction.

Sidewall functionalization of single-wall carbon nanotubes with arylradicals has been reported when aryl diazonium salts were reducedelectrochemically using single-wall carbon nanotube buckypaper aselectrodes. (see Bahr). Functionalization has also been reported usingdiazonium compounds generated in situ. (see J. L. Bahr, et al., Chem.Mater. 2001, 13, 3823-3824). Radical addition of perfluoroalkyl groupsgenerated by photolysis of corresponding species possessing acarbon-iodine bond has also been reported by Holzinger. Other examplesof sidewall functionalization include electrochemical reductive andoxidative coupling by substituted phenylated derivatives (see S. E.Kooi, et al., Angew. Chem. Int. Ed., 2002, 41, 1353-1355) andelectrophilic addition of chloroform followed by hydrolysis andesterification (see N. Tagmatarchis, et al., Chem. Commun., 2002,2010-2011). Dissolved lithium metal in liquid ammonia (Birch reduction)was used to hydrogenate SWNT. (see S. Pekker, et al., J. Phys. Chem. B.,2001, 105, 7938-7943.)

There remains, however, a need for a convenient and efficient method fornon-destructively functionalizing single-wall carbon nanotubes with avariety of functional groups, especially organic groups which can beused for further reactions, so as to be bound or otherwise associatedwith polymers, biomedical species, and other materials for a particularend-use application.

SUMMARY OF THE INVENTION

In one embodiment, the present invention involves a convenient,economical method for non-destructively functionalizing the sidewall ofsingle-wall carbon nanotubes (SWNT) or fluorinated single-wall carbonnanotubes utilizing peroxides. In one embodiment, the method forsidewall functionalizing a single-wall carbon nanotube comprisesdecomposing a diacyl peroxide in the presence of carbon nanotubeswherein the decomposition generates carbon-centered free radicals thatreact and form covalently bonds with carbon in the single-wall carbonnanotube wall to form a single-wall carbon nanotube sidewallfunctionalized with at least one organic group through a carbon bond tothe nanotube. An acyl peroxide, also known as a diacyl peroxide, is acompound with a structure of the type RC(O)OOC(O)R′, where R and R′groups can be either alkyl or aryl. In one embodiment, the acyl peroxideis an aroyl peroxide wherein the R or R′ group comprises an aromaticcomponent. In another embodiment, the acyl peroxide is an aroyl peroxideand comprises benzoyl peroxide, which, upon decomposition, liberatescarbon dioxide and generates phenyl radicals that attach to thesidewalls of the nanotubes to form sidewall phenylated single-wallcarbon nanotubes.

In an embodiment of the present invention, the acyl peroxide is aperoxydicarbonate. A peroxydicarbonate is an acyl peroxide wherein the Rand R′ are alkoxy groups. In another embodiment, the acyl peroxide hasterminal carboxylic acid groups. Examples of acyl peroxides withterminal carboxylic acid groups include, but are not limited to,succinic acid peroxide and glutaric acid peroxide. In this embodiment,the peroxide decomposes, liberating carbon dioxide and formingcarbon-centered free radicals that attach to the sidewalls of thenanotubes and provide organic groups with terminal carboxylic acidfunctionality that are then available for further reaction. An exampleof further reactions with a carboxylic acid functional group include,but are not limited to, reaction with a chlorinating agent (such asthionyl chloride) to form an acylchloride and further reaction of theacylchloride with an amine to form an amide or a diamine to form a amidelinkage with a terminal amine group.

Suitable peroxides for use with this invention include, but are notlimited to, acyl peroxides, wherein the R and R′ organic groups can bethe same or different. When the R and R′ groups are the same, the acylperoxide is a symmetrical peroxide. When the R and R′ groups aredifferent, the acyl peroxide is an asymmetrical peroxide. The R and R′organic groups of the acyl peroxide can comprise R and R′ groups of theform including, but not limited to, alkyl, cyclic, aryl and combinationsthereof.

In one embodiment, the method involves generating a phenyl radical, suchas through the decomposition of the acyl peroxide, benzoyl peroxide, andreacting the phenyl radical with an organic iodide to generate acarbon-centered radical from the organic group that was bonded to theiodine. The organic group can be alkyl or aryl. When the organic groupis an alkyl, the alkyl radical reacts with the single-wall carbonnanotube to form a covalent bond with the sidewall of the single-wallcarbon nanotube to form a sidewall-alkylated single-wall carbonnanotube. The alkyl iodide can comprise various alkyl groups, including,but not limited to, a hydrocarbon alkyl group, an alkyl amide, an alkylamine, alkyl halide, an alkyl cyanide, an alkyl ether, an alkylthioether, a trialkyl phosphine, an alkyl carboxylic acid, an alkylcarboxylate and combinations thereof. Generally, the number of carbonsin the alkyl group of the alkyl iodide is in the range of 1 to about 30.All radicals attach by a carbon linkage to the carbon nanotube.

In another embodiment, carbon-centered free radicals can be generatedfrom hydroxyl radicals and organic sulfoxides. In this embodiment,hydroxyl radicals are formed from Fenton's reagent, wherein hydrogenperoxide and divalent iron react. The hydroxyl radicals further reactwith an organic sulfoxide of the form R—S(O)—R′, to form .R and .R′carbon-centered free radicals. The .R and .R′ free radicals then reactwith single-wall carbon nanotubes and bond to the sidewall to formsingle-wall carbon nanotubes with R and R′ groups attached to theirsidewall. R and R′ can be the same or different. The R and R′ groups canalso be aryl groups, such as, but not limited to phenyl groups and otheraromatic groups. The reaction generates carbon-centered free radicalsfrom an organic sulfoxide and will be referred to as a “Minisci” or“Minisci-type” reaction. The carbon-centered free radicals can begenerated from the organic sulfoxide using Fenton's reagent, whichgenerates hydroxyl radicals from the reaction of hydrogen peroxide andan Fe(II) catalyst, such as FeSO₄.

The method of this invention applies to both single-wall and multi-wallcarbon nanotubes to functionalize the exterior wall of the carbonnanotube without destroying the carbon wall structure. Besides pristine,underivatized nanotubes, sidewall-fluorinated carbon nanotubes can alsobe used as the initial nanotubes for further sidewall derivatization.Although not meant to be bound by theory, the presence of sidewallfluorine groups appears to make the nanotubes more reactive to sidewallderivatization.

Certain properties of the nanotubes can be imparted to other materialsmore effectively when the nanotubes have sidewall functionality. Thesidewall-functionalized nanotubes can be made more compatible anddispersible in other materials. In one embodiment, the non-destructivesidewall functionalization of single-wall carbon nanotubes enables theincorporation of the nanotubes into composite materials by reacting withan appropriate functionality attached to the sidewall of the single-wallcarbon nanotube. In another embodiment, the sidewall functionalizationof single-wall carbon nanotubes with aromatic groups or long chain alkylgroups, oligomers, or polymers can make the single-wall carbon nanotubesmore compatible with various materials, such as, but not limited to,polymers and other organic materials, and enable composites with highdispersability and enhancement of the materials' properties. Because thestructure of the nanotubes is still intact after the sidewallfunctionalization, the mechanical properties of the nanotubes cancontribute to the strength and modulus of the composite materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reaction of one embodiment of the present inventionwherein benzoyl peroxide 1 is used to functionalize the sidewall of asingle-wall carbon nanotube (SWNT) and form sidewall-phenylated-SWNT 2.

FIG. 2 shows a reaction scheme of one embodiment of the presentinvention wherein a phenyl radical, such as from benzoyl peroxide 1reacts with RI 3, where examples of R are given in the list of 4a though4j, to form an .R carbon-centered free radical, which attaches to theSWNT sidewall to form sidewall-derivatized R-SWNT 4.

FIG. 3 shows a reaction scheme of one embodiment of the presentinvention wherein hydrogen peroxide in the presence of Fe⁺² forms ahydroxyl free radical. The radical is further reacted with dimethylsulfoxide to form a methyl radical. The methyl radical attaches to theSWNT sidewall to form sidewall-methylated-SWNT 5a.

FIG. 4 shows a reaction scheme of one embodiment of the presentinvention wherein dimethyl sulfoxide in the presence of Fe⁺² andhydrogen peroxide generates a methyl radical. The methyl radical in thepresence of an iodo-R compound RI generates iodomethane and an .Rcarbon-centered free radical.

FIG. 5 shows a reaction scheme of one embodiment of the presentinvention wherein an organic sulfoxide (R—S(O)—R′) in the presence ofFe⁺² and hydrogen peroxide generates .R and .R′ radicals, which furtherreact with SWNT to form R- and R′-sidewall functionalized SWNT.

FIG. 6 shows a reaction scheme of one embodiment of the presentinvention wherein a dicarboxylic acid acyl peroxide such as 6a or 6b, inthe presence of heat, liberates CO₂ and generates a carbon-centered freeradical which bonds to the sidewall of a single-wall carbon nanotube toform sidewall derivatized SWNT with organic groups having terminalcarboxylic acid groups, such as 7a or 7b.

FIG. 7 shows a reaction scheme of one embodiment of the presentinvention wherein a single-wall carbon nanotube side-wall functionalizedwith alkyl having a terminal carboxylic acid group 7a is reacted withthionyl chloride to form an alkyl acyl chloride sidewall-functionalizedSWNT which is further reacted with ethylene diamine to form a sidewallalkyl amide functionalized SWNT 8, wherein the sidewall organic grouphas a terminal amine.

FIG. 8 shows a reaction scheme of one embodiment of the presentinvention wherein a single-wall carbon nanotube sidewall functionalizedwith an alkyl having a terminal carboxylic acid group 7b is reacted withthionyl chloride to form an alkyl acyl chloride sidewall-functionalizedSWNT which is further reacted with 4,4′-methylenebis(cyclohexylamine) toform a side-wall alkyl amide functionalized SWNT 9, wherein the sidewallorganic group has a terminal amine. The subscript “x” indicates avariable number of sidegroup attachments.

FIG. 9 shows a reaction scheme of one embodiment of the presentinvention wherein a single-wall carbon nanotube side-wall functionalizedwith an alkyl having a terminal carboxylic acid group 7b is reacted withthionyl chloride to form a terminal acyl chloridesidewall-functionalized SWNT which is further reacted withdiethyltoluenediamine to form an aryl side-wall functionalized SWNT 10,wherein the aryl sidewall function has an amide linkage and a terminalamine. The subscript “x” indicates a variable number of sidegroupattachments.

FIG. 10 shows a reaction scheme wherein single-wall carbon nanotubesthat have been sidewall functionalized with an ester are saponified withsodium hydroxide and neutralized with hydrochloric acid to formsingle-wall carbon nanotubes that have alkyl groups with terminalcarboxylic acid functionality, which are further chlorinated withthionyl chloride to form an acyl chloride, which is further reacted withoctadecyl amine to form an alkyl amide group that is bonded to thesingle-wall carbon nanotube sidewall, where the subscript “x” indicatesa variable number of sidegroup attachments.

FIG. 11 shows Raman spectra for certain embodiments of the presentinvention, including spectra for the following pristine andsidewall-derivatized SWNT, wherein the subscript “x” indicates a numberof sidegroup attachments which is variable depending on the sidegroupand conditions of preparation:

FIG. 11A for pristine SWNT;

FIG. 11B for SWNT-(CH₂CH₂COOH)_(x);

FIG. 11C for SWNT-(CH₂CH₂CH₂COOH)_(x); and

FIG. 11D for SWNT-(CH₂CH₂COOH)_(x) after heating to 800° C. in argon perTGA.

FIG. 12 shows FTIR spectra for certain embodiments of the presentinvention including spectra for the following sidewall-derivatized SWNT,wherein the subscript “x” indicates a number of sidegroup attachmentswhich is variable depending on the sidegroup and conditions ofpreparation:

FIG. 12A for SWNT-(CH₂CH₂COOH)_(x);

FIG. 12B for SWNT-(CH₂CH₂CONHCH₂CH₂NH₂)_(x);

FIG. 12C for SWNT-(CH₂CH₂CONHC₆H₁₀CH₂C₆H₁₀NH₂)_(x); and

FIG. 12D for SWNT-(CH₂CH₂CONHC₆H(C₂H₅)₂CH₃NH₂)_(x).

FIG. 13 shows TGA/MS of SWNT-(CH₂CH₂COOH)_(x), with peak evolutions dueto the following sidegroup fragments and associated atomic mass:

-   -   a: —CH₂CH₃ (mass: 29),    -   b: —COO— (mass: 44),    -   c: —COOH (mass: 45),    -   d: —CH₂CH₂COOH (mass: 73) and    -   e: —CH₂CH₂COO— (mass: 72).

FIG. 14 shows ¹³C NMR spectra for certain embodiments of the presentinvention including spectra for the following pristine andsidewall-derivatized SWNT,

wherein the subscript “x” indicates a number of sidegroup attachmentswhich is variable depending on the sidegroup and conditions ofpreparation:

FIG. 14A for pristine HIPCO SWNT;

FIG. 14B for SWNT-(CH₂CH₂CH₂COOH)_(x);

FIG. 14C for SWNT-(CH₂CH₂COOH)_(x); and

FIG. 14D for SWNT-(CH₂CH₂CONHCH₂CH₂NH₂)_(x).

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The carbon nanotubes, according to the present invention, can be made byany known method. Single-wall carbon nanotubes are generally made inhigh-temperature processes using a carbon source and a metalliccatalyst, typically comprising Group VIb and/or Group VIIIb transitionmetals. Methods for synthesizing single-wall carbon nanotubes include DCarc processes; laser vaporization of graphite doped with transitionmetal atoms; high temperature, high pressure gas-phase synthesesinvolving a carbon-containing feedstock gas, such as carbon monoxide;and a volatile transition metal catalyst precursor, and chemical vapordeposition (CVD) processes in which single-wall carbon nanotubes areformed from a carbon-containing gas on nanometer-scale metal catalystparticles, which can be supported on a substrate or catalyst support.The process type and operating conditions will produce single-wallcarbon nanotubes having a particular distribution of diameters andlengths. Generally, the lengths of as-produced single-wall carbonnanotubes are in excess of about 50 nm, and more typically, greater thanabout 100 nm. Commonly, single-wall carbon nanotubes have lengths in therange of about 1 and about 10 microns.

All known methods of synthesizing single-carbon nanotubes also produce adistribution of reaction products, including, but not limited to,single-wall carbon nanotubes, amorphous carbon, metallic catalystresidues, and, in some cases, multi-wall carbon nanotubes. Thedistribution of reaction products will vary depending on the process andthe operating conditions used in the process.

The nanotubes can be optionally purified. In one embodiment, single-wallcarbon nanotube powder is purified to remove non-nanotube carbon, suchas amorphous carbon and metallic catalyst residues. Metals, such asGroup VI and/or VIII are possible catalysts for the synthesis ofsingle-wall carbon nanotubes, and the metallic residues after catalysismay be encapsulated in non-nanotube carbon, such as graphitic shells ofcarbon. The metallic impurities may also be oxidized through contactwith air or by oxidation of the non-nanotube carbon during purification.

Purification can be done by any known means. Procedures for purificationof carbon nanotubes are related in International Patent Publications“Process for Purifying Single-Wall Carbon Nanotubes and CompositionsThereof,” WO 02/064,869 published Aug. 22, 2002, and “Gas Phase Processfor Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO02/064,868 published Aug. 22, 2002, and incorporated by reference hereinin their entirety. In an embodiment of the present invention, thenanotubes are purified by heating at 250° C. in air saturated with watervapor. The heating is done for a length of time so as to oxidize atleast some of the non-nanotube carbon, and, may, to some extent, oxidizethe metal impurities. The oxidation temperature can be in the range ofabout 200° C. to about 400° C., preferably about 200° C. to about 300°C. The oxidation can be conducted in any gaseous oxidative environment,which can comprise such oxidative gases as such as oxygen, air, carbondioxide, and combinations thereof. The concentration of the oxidativegases can be adjusted and controlled by blending with nitrogen, an inertgas, such as argon, or combinations thereof. The duration of theoxidation process can range from a few minutes to days, depending on theoxidant, its concentration, and the oxidation temperature.

After oxidatively heating the nanotubes, the nanotubes are treated withacid to remove metallic impurities and to form a slurry of nanotubes inthe acid. The acid can be a mineral acid, an organic acid, orcombinations thereof. Examples of acids that could be used to treat andslurry the nanotubes include, but are not limited to, hydrochloric acid,hydrofluoric acid, hydrobromic acid, hydroiodic acid, sulfuric acid,oleum, nitric acid, citric acid, oxalic acid, chlorosulfonic acid,phosphoric acid, trifluoromethane sulfonic acid, glacial acetic acid,monobasic organic acids, dibasic organic acids, and combinationsthereof. The acid used can be a pure acid or diluted with a liquidmedium, such as an aqueous and/or organic solvent. Generally, an aqueoussolvent is preferred. Concentrated aqueous hydrochloric acid ispreferred for removing metallic impurities.

After the metallic impurities react with the acid, the nanotubes arefiltered and washed to remove the acid from the nanotubes. Examples ofwashing media that could be used to wash the acid from the nanotubesinclude, but are not limited to, water, alcohols, aldehydes, ketones,dilute alkaline solutions, ammonium hydroxide, primary, secondary andteriary amines, oxygenated hydrocarbons, hydroxylated hydrocarbons,organic aromatics and combinations thereof.

After the nanotubes are washed, they can, optionally, be dried, such asin a vacuum oven or an inert environment, such as in nitrogen or aninert gas atmosphere, with or without heat.

Furthermore, the single-wall carbon nanotubes can, optionally, besidewall fluorinated. Sidewall fluorinated single-wall carbon nanotubesare designated “F-SWNT” herein. F-SWNT can be prepared, preferably withpurified single-wall carbon nanotubes, by direct fluorination using suchprocedures as given in Z. Gu, et al., Nano Lett., 2002, 2, 1009. TheSWNT can be fluorinated to any level up to the theoretical limit of onefluorine to two carbon atoms on the SWNT and still have the wallstructure intact. The stoichiometry of maximum SWNT fluorination is C₂F.

Sidewall Functionalization of Carbon Nanotubes Utilizing Acyl and AroylPeroxides

In one embodiment of the present invention, the decomposition of acylperoxides is used to generate carbon-centered free radicals, whichnon-destructively add organic groups through a carbon linkage to thesidewall and/or closed end cap of a carbon nanotube.

Acyl peroxides, or alternatively, diacyl peroxides, have the chemicalformula R—C(O)O—O(O)C—R′. The O—O bond is very weak and under suitableconditions, the O—O bond can readily undergo bond homolysis to form anintermediate carboxyl radical which decarboxylates to produce carbondioxide and carbon-centered radicals, such as .R, .R′, or a combinationthereof. The R and R′ groups can be the same or different. The R and R′can be alkyl, aryl, or cyclic. In addition, the R and R′ groups can haveterminal functional groups and contain heteroatoms, other than carbonand hydrogen. Acyl peroxides are conveniently and economicallyavailable, or can be synthesized, with a wide variety of R and R′groups.

In one embodiment of the present invention, an alkyl group, such as anundecyl (C₁₁H₂₃) group can be bonded to the sidewalls of single-wallcarbon nanotubes using undecyl groups generated by the decomposition ofthe acyl peroxide, also known as lauroyl peroxide(C₁₁H₂₃—C(O)OO(O)C—C₁₁H₂₃). Lauroyl peroxide is also known by othernomenclature, such as, but not limited to, dilauroyl peroxide anddodecanoyl peroxide. The procedures for attaching alkyl groups to thesidewall of the nanotube comprise, making a suspension of nanotubes in asuitable solvent, such as benzene. Sonication, stirring and other meansof agitation can be used to facilitate dispersion of the nanotubes. Theacyl peroxide is then added to the SWNT suspension. The nanotubes andperoxide are heated to a temperature above the decomposition temperatureof the acyl peroxide and mixed for a time effective to decompose theperoxide, generate free carbon-centered radicals and bond the freeradicals to the sidewalls of the carbon nanotubes.

Examples of other suitable acyl peroxides of the form R—C(O)O—O(O)C—R′,wherein the R and R′ are organic groups that can be the same ordifferent and can include, but are not limited to, acetyl peroxide,n-butyryl peroxide, sec-butyryl peroxide, t-butyryl peroxide, t-pentoylperoxide, iso-valeryl peroxide, furoyl peroxide, palmitoyl peroxide,decanoyl peroxide, lauroyl peroxide, diisopropyl peroxydicarbonate andbutylperoxyisopropyl carbonate. The R or R′ group can comprise a normal,branched or cyclic alkyl group wherein the number of carbons can rangefrom one to about 30, and typically, in the range of about 8 and about20. The R or R′ group can contain one or more cyclic rings, examples ofwhich are trans-t-butylcyclohexanoyl peroxide,trans-4-cyclohexanecarbonyl peroxide and cyclohexyl peroxydicarbonate,cyclopropanoyl peroxide, cyclobutanoyl peroxide and cyclopentanoylperoxide. The acyl peroxides can contain heteroatoms and functionalgroups, such as bromobutyryl peroxide, (CCl₃CO₂)₂, (CF₃CO₂)₂,(CCl₃CO₂)₂, (RO(CH₂)_(n)CO₂)₂, (RCH═CR′CO₂)₂, (RC≡CCO₂)₂, and(N≡C(CH₂)_(n)CO₂)₂, where n=1-3.

The degree of functionalization of the nanotube will depend on variousfactors, including, but not limited to, the type and structure of sidegroup, steric factors, the desired level for an intended end-use, andthe functionalization route and conditions. The generally acceptedmaximum degree of functionalization of a single-wall carbon nanotube isone functional group per two single-wall nanotube carbons.

The carbon nanotube that is to be sidewall functionalized can bepristine, i.e. without prior derivatization or functionalization, or itcan be already sidewall functionalized, such as sidewall fluorinatedsingle-wall carbon nanotubes. The rate of SWNT sidewallfunctionalization is faster, in the case of the undecyl groups, whenfluorinated (F-SWNT) is used compared to unfluorinated SWNT. When F-SWNTis sidewall functionalized with an organic group, some of the fluorineatoms may remain bonded to the nanotube, such that the resultingnanotube has both fluorine and organic substituents bonded to thesidewall.

Sidewall Functionalization Utilizing Aroyl Peroxides

In another embodiment, aryl-containing groups are bonded to the sidewallof carbon nanotubes. Aryl free radicals are generated by aromatic acylperoxides, also called aroyl peroxides, wherein at least one R or R′organic group contains an aromatic moiety. In one embodiment, phenylgroups are bonded to the sidewall of single-wall carbon nanotubes usingphenyl radicals generated by decomposition of the aroyl peroxide benzoylperoxide (C₆H₅—C(O)OO(O)C—C₆H₅). The chemical reaction scheme isdiagrammed in FIG. 1, wherein benzoyl peroxide 1 is used tofunctionalize the sidewall of a single-wall carbon nanotube (SWNT) andform sidewall-phenylated-SWNT 2. Multiple phenyl groups can be attachedto the sidewall of the nanotube. For example, in one embodiment, thedegree of functionalization of phenylated single-wall carbon nanotubeswas about one phenyl group per about 14 carbons of the single-wallcarbon nanotube.

Other aroyl peroxides include, but are not limited to, cinnamoylperoxide, bis(p-methoxybenzoyl) peroxide, p-monomethoxybenzoyl peroxide,bis(o-phenoxybenzoyl) peroxide, acetyl benzoyl peroxide, t-butylperoxybenzoate, diisopropyl peroxydicarbonate, cyclohexylperoxydicarbonate, benzoyl phenylacetyl peroxide, andbutylperoxyisopropyl carbonate. The aroyl peroxide can also includeheteroatoms, such as in p-nitrobenzoyl peroxide, p-bromobenzoyl,p-chlorobenzoyl peroxide, and bis(2,4-dichlorobenzoyl) peroxide. Thearoyl peroxide can also have other substituents on one or more aromaticrings, such as in p-methylbenzoyl peroxide, p-methoxybenzoyl peroxide,o-vinylbenzoyl benzoyl peroxide, and exo- and endo-norbornene-5-carbonylperoxide. The aromatic ring substitutions of the various groups andheteroatoms can also be in other positions on the ring, such as theortho, meta or para positions. The aroyl peroxide can also be anasymmetric peroxide and include another organic group that can be analkyl, cyclic, aromatic, or combination thereof.

In another embodiment, organic iodides, herein denoted RI, can be usedas a source of the R organic group for sidewall functionalization of thewall of the carbon nanotubes. The R group can be an alkyl group, arylgroup, cyclic group, or combination thereof. The R group can containfunctional moieties, such as, but not limited to, carboxylic acid,carboxylates, cyanide groups, nitro groups, esters, ethers, ketones,amides, heteroatoms, such as nitrogen, oxygen, halogens, andcombinations thereof. Generally, the number of carbons in the organiciodide is in the range of 1 to about 30. The R—I bond is a carbon-iodinebond, such that the free radical generated from RI is a carbon-centeredfree radical. In this embodiment, RI is mixed with a peroxide, such asbenzoyl peroxide. After the benzoyl peroxide decomposes to phenylradicals, a free radical displacement of iodide by the phenyl radicalgenerates an .R carbon-centered free radical, which then bondscovalently to the carbon nanotube sidewall. In embodiments wherein thecarbon nanotubes are single-wall carbon nanotubes, the result is asidewall R-derivatized SWNT. FIG. 2 shows a reaction scheme wherein aphenyl radical (such as from benzoyl peroxide) undergoes a radicaldisplacement of iodide from RI 3 to generate an .R carbon-centered freeradical and wherein the .R free radical bonds to the SWNT sidewall andforms sidewall derivatized SWNT 4. Various organic R groups are shown inFIG. 2 as 4a through 4j. “THP” in 4g means “tetrahydropyran”.

Solid State and Liquid State Preparation Procedures

In another embodiment of the present invention, the proportion ofsingle-wall carbon nanotubes used is approximately one to two times thatof the acyl peroxide on a weight basis. For solid state reactions, amechanically-ground mixture of reactants is used in a sealed reactor,such as a stainless steel reactor. The temperature of the reactor isheated above the decomposition temperature of the peroxide, such as atemperature of about 200° C. for a time sufficient to carry out thereaction to the desired state of completion. The reaction time will varydepending on the acyl peroxide used. A typical reaction time can beabout 12 hours. Solid state reactions can be done with solid peroxidesand carbon nanotubes. The method is convenient and no solvent dispersionor solvent removal is required.

For solution phase reactions, the single-wall carbon nanotubes aredispersed in a suitable solvent, such as, for example, benzene,o-dichlorobenzene, or nitrobenzene.

Dispersing the nanotubes in solvent can be facilitated by sonication orultrasonication. The time required to disperse the nanotubes isdependent on the solvent and the amount and type of nanotubes beingdispersed. Generally, the nanotubes can be dispersed in about 30 minutesto about 2 hours, although longer mixing times may be required,depending on the dispersing apparatus, among other factors. Afterdispersing the nanotubes in solvent, the acyl peroxide is added and themixture is refluxed under an environment of nitrogen or an inert gas,such as argon. The solvent is also selected such that the refluxing isdone above the decomposition temperature of the selected peroxide. Forexample, when using o-dichlorobenzene as a solvent and either benzoylperoxide or lauroyl peroxide, the reflux conditions are in a temperaturerange of about 80° C. to about 100° C. for a time in a range of about 3hours to about 120 hours effective for the decomposition of the acylperoxide to form carbon-centered radicals. For example, in oneembodiment, purified single-wall carbon nanotubes are sonicated for 30minutes to suspend the nanotubes in benzene. The suspension is thenheated in the presence of benzoyl peroxide to 75° C. and held attemperature for 24 hrs under an argon atmosphere to bond phenyl groupsto the single-wall carbon nanotube sidewalls.

After the sidewall functionalization reaction is complete, the sidewallfunctionalized single-wall carbon nanotubes can be isolated fromunreacted peroxides and by-products by washing with solvent. Forexample, sidewall-functionalized SWNT can be filtered from the unreactedproducts and by-products, and washed with a solvent, such as chloroform.The nanotubes can then be dried, such as in a vacuum oven at about 100°C. overnight.

Preparation of SWNT Sidewall Functionalized with Alkyl Radicals fromOrganic Iodides

In another embodiment, the acyl peroxide, benzoyl peroxide, is used withR-iodide (RI) to attach organic R groups other than phenyl to the carbonnanotube sidewall. In this embodiment, benzoyl peroxide is decomposed tocarbon dioxide and phenyl radicals. The phenyl radicals react with theR-iodide via an iodide displacement to form .R radicals. The .R radicalsbond to the nanotubes to form carbon nanotubes with R groups bonded tothe sidewall of the nanotubes. The chemical reaction scheme for thismethod of preparing sidewall functionalized SWNT is shown in FIG. 2,wherein 3 is RI and 4 represents sidewall functionalized R-SWNT.Examples of R groups are also shown in FIG. 2 as 4a through 4j. Thegroups represented are n-octadecyl 4a, n-propyl 4b, sec-butyl 4c, ethylamide 4d, n-propyl chloride 4e, actetonitrile 4f, and n-propyltetrahydropyran ether 4g, ethyl acetate 4h, poly(ethyleneglycol)-n-butyl ether 41, and neopentyl 4j. Examples of other R groupsthat may be attached to the sidewall of single-wall carbon nanotubes inthis manner, include, but are not limited to linear, cyclic and branchedalkyl groups, (an example of a branched alkyl group is a neopentylgroup), polymeric groups, such as polyethylene glycol, polyolefins,polyesters, polyurethanes, functionalized polymer groups, such aspolyethylene glycol n-butyl ether groups, functional groups such asethers, alcohols, carboxylic acid groups, carboxylates, aromatic groups,aromatic groups substituted with functional groups, and/or heteroatoms,such as halides, nitro groups, amino groups, and combinations thereof.

Preparation of SWNT Sidewall Functionalized with Methyl Groups

Methyl radicals can be generated from dimethyl sulfoxide by the methodof Minisci (see Fontana, F; Minisci, F.; and Vismara, E. TetrahedronLett. 1988, 29, 1975-1978, “Minisci”, incorporated herein by reference)by reaction with hydroxyl radicals. A convenient source of hydroxylradicals can be generated using Fenton's reagent, which includeshydrogen peroxide and a divalent iron catalyst. The methyl radicalsgenerated from the dimethyl sulfoxide and hydroxyl radicals can bond tothe carbon nanotube wall to form sidewall methylated carbon nanotubes.One embodiment for methylating the sidewall of single-wall carbonnanotubes is diagrammed in FIG. 3. As shown in FIG. 3, hydrogenperoxide, in the presence of Fe⁺², forms a hydroxyl free radical. Theradical is further reacted with a dimethyl sulfoxide to form a methylradical. The methyl radical attaches to the SWNT sidewall to form asidewall-methylated-SWNT 5a. Although the methyl radicals effectivelybond to the sidewall of the SWNT, the resulting methylated SWNT has lesssolubility in most organic solvents than SWNT alkylated with groupscomprising more carbon atoms or larger functional groups.

Preparation of Methyl Radicals Using the Minisci Route and the Formationof Other Organic Radicals Using Radical Displacement of Iodine

Because the methyl radical is the least stable alkyl radical, it can beused to selectively generate other alkyl radicals. The methyl radicalsgenerated using the Minisci method can also be used to generate otheralkyl radicals using organic iodides (RI) as the source of other alkylor aryl groups. This process, diagrammed in FIG. 4, offers another routeto other free radicals and another embodiment for adding functionalgroups to the carbon nanotube sidewall. FIG. 4 shows a reaction schemewherein dimethyl sulfoxide, in the presence of Fe⁺² and hydrogenperoxide, generates a methyl radical. The methyl radical in the presenceof an iodo-R compound RI generates iodomethane and an .R carbon-centeredfree radical. Examples of R groups in RI include, but are not limitedto, those previously listed herein.

Preparation of SWNT Sidewall Functionalized with Various Organic RGroups Using the Minisci Method of Generating Free Radicals

Alkyl and aryl radicals can be generated using the Minisci method usingsulfoxides with various alkyl and/or aryl groups. In this embodiment,other alkyl groups can be attached to the SWNT sidewall without theiodide replacement reaction. In this embodiment, sulfoxides, which havethe form R—S(O)—R′, where —R and —R′ can be the same or different, canalso be used to generate various carbon radicals without the use organiciodides. The R groups can be alkyl or aromatic or a combination thereof.This process, diagrammed in FIG. 5, offers another route to other freeradicals and another embodiment for adding functional groups to thecarbon nanotube sidewall. FIG. 5 shows a reaction scheme wherein anorganic sulfoxide (R—S(O)—R′), in the presence of Fe⁺² and hydrogenperoxide, generates .R and .R′ radicals, which further react with SWNTto form R— and R′-sidewall functionalized SWNT. Examples of R groupsshown in FIG. 5 include methyl groups 5a, n-propyl groups 5b, isopropylgroups 5c, n-butyl groups 5d, sec-butyl groups 5e, and phenyl groups 5f.The R or R′ group generally can comprise a number of carbons in therange of 1 and about 30.

Preparation of SWNT Sidewall Functionalized Organic Moieties withTerminal Carboxylic Acid Groups

In another embodiment, alkyl groups terminated with the carboxylic acidfunctionality are attached to the sidewalls of the single-wall carbonnanotubes. FIG. 6 shows an embodiment wherein a dicarboxylic acid acylperoxide such as 6a or 6b, in the presence of heat, liberates CO₂ andgenerates a carbon-centered free radical which bonds to the sidewall ofa single-wall carbon nanotube to form sidewall derivatized SWNT withorganic groups having terminal carboxylic acid groups, such as 7a or 7b.In one embodiment diagrammed in FIG. 6, organic acyl peroxides ofdicarboxylic acids, such as HO(O)C(CH₂)_(n)C(O)OO(O)C(CH₂)_(n)C(O)OH(where 6a, n=2, is succinic acid peroxide; and 6b, n=3, is glutaric acidperoxide) are used to functionalize the sidewall of single-wall carbonnanotubes with functional groups of the form HO(O)C(CH₂)_(n)—, where thesidewall functionalized SWNT for n=2 using 6a is represented by 7a andfor n=3 using 6b is represented by 7b. In another embodiment, phthalicacid peroxide is decomposed to an organic radical, wherein the radicalbonds with wall of the carbon nanotube and the resulting organicsidegroup has a terminal carboxylic acid group.

Acyl peroxides have the form, RC(O)OO(O)CR, where R can be aliphatic,aromatic or another group, and readily decompose to release carbondioxide and form free radicals R upon mild heating. Succinic acidperoxide, 6a, decomposes to form 3-carboxyl-propionyl-oxyl radicals,which can subsequently release CO₂ to yield another radical,2-carboxyl-ethyl. Glutaric acid peroxide, 6b, yields a 3-carboxyl-propylradical via a similar route. These carboxyl-alkyl radicals, generated insitu from corresponding peroxides 6a and 6b, react with single-wallcarbon nanotubes to produce sidewall acid-functionalizedSWNT-derivatives 7a and 7b, as diagrammed in FIG. 6. The subscript “x”in the functionalized SWNT derivatives indicates the attachment of avariable number of sidewall groups to the nanotube.

Reactions of Carboxylic Acid Sidewall Functionalized SWNT with Aminesand Diamines

Single-wall carbon nanotubes with sidewall alkyl groups having terminalcarboxylic acid functionality, such as 7a and 7b, shown in FIG. 6, canfurther be reacted to yield nanotubes with other reactive functionality.For example, amide derivatives can be made by reacting the carboxylicacid functionality with a chlorinating agent, such as thionyl chloride,and subsequently with an amine compound. Other possible chlorinatingagents, include, but are not limited to phosphorous trichloride,phosphorous pentachloride, and oxalyl chloride (C₂O₂Cl₂). To give theSWNT side group a terminal amine, a diamine can be used. Examples ofsuitable diamines are ethylene diamine, 4,4′methylenebis(cyclohexylamine), propylene diamine, butylene diamine,hexamethylene diamine and combinations thereof. A reaction scheme of oneembodiment of the present invention using the diamine, ethylene diamineis shown in FIG. 7. FIG. 7 shows a reaction scheme wherein a single-wallcarbon nanotube sidewall functionalized with an alkyl having a terminalcarboxylic acid group 7a is reacted with thionyl chloride to form analkyl acyl chloride sidewall-functionalized SWNT which is furtherreacted with ethylene diamine to form a sidewall alkyl amidefunctionalized SWNT 8, wherein the sidewall organic group has a terminalamine. The subscript “x” in 8 indicates a variable number of sidegroupson the nanotube.

In another embodiment, diagrammed in FIG. 8, a diamine having a cyclicring, 4,4′-methylenebis(cyclohexylamine), is used to give a terminalamine functionality to group on the SWNT sidewall. FIG. 8 shows areaction scheme wherein a single-wall carbon nanotube sidewallfunctionalized with an alkyl having a terminal carboxylic acid group 7bis reacted with thionyl chloride to form an alkyl acyl chloridesidewall-functionalized SWNT which is further reacted with4,4′-methylenebis(cyclohexylamine) to form a sidewall alkyl amidefunctionalized SWNT 9, wherein the sidewall organic group has a terminalamine. The subscript “x” in 9 indicates a variable number of sidegroupson the nanotube.

In another embodiment, an aryl diamine, diethyltoluenediamine, is usedin the reaction scheme diagrammed in FIG. 9. FIG. 9 shows a reactionscheme wherein a single-wall carbon nanotube sidewall functionalizedwith an alkyl having a terminal carboxylic acid group 7b is reacted withthionyl chloride to form a terminal acyl chloridesidewall-functionalized SWNT which is further reacted withdiethyltoluenediamine to form an aryl sidewall functionalized SWNT 10,wherein the aryl sidewall function an amide linkage and a terminalamine. The subscript “x” in 10 indicates a variable number of sidegroupson the nanotube.

The method of this invention is also generally applicable to multi-wallcarbon nanotubes. In the case of multi-wall carbon nanotubes, the outersidewall can be functionalized in the same manner as that of the singlewall of a single-wall carbon nanotube.

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventors to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

Example 1

This example describes the procurement, preparation and purification ofthe single-wall carbon nanotubes. HIPCO® single-wall carbon nanotubes,having an average diameter of about 1 nm, were obtained from RiceUniversity (HIPCO is a trademark registered to Carbon Nanotechnologies,Inc., Houston, Tex.) To remove iron, used as a catalyst, the nanotubeswere purified using a gaseous wet air oxidation followed by an aqueoushydrochloric acid rinse according to the procedures of Chiang, I. et al.J. Phys. Chem. B, 2001, 105, 8297-8301 (“Chiang”). Following thepurification, the nanotubes were annealed at 800° C. for 2 hours inargon.

Example 2

This example demonstrates a procedure for functionalizing the sidewallsof single-wall carbon nanotubes using the acyl peroxide, benzoylperoxide.

Purified SWNT (50 mg) was sonicated in benzene (65 ml) for 2 hours toform a SWNT suspension. 6.7 g benzoyl peroxide, which decomposes undermild conditions (75-80° C.) to form carbon dioxide and phenyl radicals,as shown in FIG. 1, was added to the SWNT suspension. The heterogeneousmixture was stirred under argon at 70-80° C. overnight to decompose thebenzoyl peroxide and form sidewall-phenylated-SWNT (Ph-SWNT). During theheating, the benzoyl peroxide decomposed to phenyl radicals and carbondioxide, and the phenyl radicals attached to the sidewalls of the SWNTto form sidewall-derivatized phenyl-SWNT (Ph-SWNT). 50 ml ethanol wasthen added to precipitate the dissolved Ph-SWNT. The mixture was thenfiltered through a 0.2 μm PTFE (polytetrafluoroethylene) filter andwashed with ethanol. The nanotubes that were collected from the filterwere washed by suspending the nanotubes in 100 ml ethanol, sonicatingthe mixture for 30 min and filtering again. The ethanol-washingprocedure was repeated twice, except that hot ethanol was used in thelast washing. The ethanol-washed Ph-SWNT was dried under vacuum for 4hours at 70° C.

The degree of phenyl-group functionalization was estimated by TGA(thermal gravimetric analysis) to be about one phenyl group for about 14carbon atoms in the nanotube framework. (See FIG. 1 which gives adiagram of the chemical reaction. See also Example 3 in Table 1 below.)Increasing the peroxide concentration did not substantially increase thedegree of phenyl functionalization, suggesting that steric effects maybe a limiting factor in the degree of phenyl-group functionalization.

The phenyl-functionalization caused the nanotubes to exhibit remarkablyimproved solubility and/or dispersability in CHCl₃, CH₂Cl₂, DMSO(dimethylsulfoxide), DMF (dimethylformamide) and 1,2-dichlorobenzenesolvents. Although the solubility of the phenylated nanotubes also washigher than pristine nanotubes in benzene, toluene and THF(tetrahydrofuran), the increase in solubility was not as pronounced inCHCl₃, CH₂Cl₂, DMSO, DMF and 1,2-dichlorobenzene. The phenylatednanotubes were insoluble in n-hexane and ether.

Example 3

This example demonstrates a general method for functionalizing thesidewalls of single-wall carbon nanotubes with R groups other thanphenyl groups using benzoyl peroxide as an initial source of freeradicals that react with R-iodides. To attach R groups other than phenylto the SWNT sidewall, R-iodide, (RI) (2.5 equivalents per mole ofbenzoyl peroxide) was used with benzoyl peroxide. The R groups used inthis example are given in Table 1, shown below. The correspondingR-iodide compounds were obtained from Aldrich, except for the iododerivative of poly(ethylene glycol)-n-butyl ether (See 4i in Table 1),which was prepared as follows according to procedures adapted from S.Arndt, et al., Chem. Eur. J. 2001, 7, 993-1005. To a solution ofimidazole (3.3843 g, 49.71 mmol) and PPh₃ (5.214 g, 19.88 mmol) in 110ml CH₂Cl₂ at 0° C., iodine (5.467 g, 21.54 mmol) was added. The solutionwas stirred for 5 min, and poly(ethylene glycol)-n-butyl etherHO(CH₂CH₂)_(m)—CH₂CH₂CH₂CH₃ (3.4135 g, 16.57 mmol, average MW about 206,m≈3) dissolved in 20 ml CH₂Cl₂, was then added slowly. The mixture wasstirred for 4 hrs with exclusion of light. The reaction was thenquenched by adding 20 ml saturated aqueous solution of sodium sulfiteNa₂SO₃. The aqueous layer was extracted with ethyl ether and the extractdried over MgSO₄. The solvents were removed by rotary evaporator. Thecrude iodo derivative of poly(ethylene glycol)-n-butyl ether waspurified by column chromatography (3.403 g, 10.77 mmol, yield 65%).

The reaction scheme for the preparation of sidewall functionalizedR-SWNT is shown in FIG. 2. The benzoyl peroxide is decomposed to phenylradicals, which, in turn, react with the R-iodide to form organic .Rfree radicals, which, in turn, bond to the sidewall of the single-wallcarbon nanotube. The general procedures, which follow, were used withdifferent RI compounds.

20 mg purified single-wall carbon nanotubes (1.6 mmols carbon) weresuspended in 30 ml benzene and sonicated for 30 min. in a bath sonicator(Cole Parmer) to form a SWNT-benzene suspension. R-iodide (4.0 mmol) and403 mg benzoyl peroxide (1.6 mmol) were added to the SWNT-benzenesuspension. The resulting mixture was heated under argon at 75° C. for24 hours with stirring. After the heating, the mixture was diluted with100 ml benzene and filtered through a 0.2 μm PTFE(polytetrafluoroethylene) membrane. The sidewall-functionalized R-SWNTwas caught on the filter and washed with copious amounts of benzene. Thewashing procedure (filtering and washing) was repeated twice withbenzene and once each with acetone and methanol. The washed R-SWNT wasfinally removed from the filter and dried under vacuum at 80° C. for 12hours. Even though benzoyl peroxide was used as the initial radicalsource, no evidence of phenyl groups was evident on the varioussidewall-functionalized R-SWNT. The R groups shown are shown in Table 1,along with the TGA (thermogravimetric analysis) of the sidewallR-functionalized SWNT. TGA was used to determine the weight loss due tothe evolution of the particular R group and the associated estimate ofthe ratio of carbons associated with SWNT to the number of R groupsbonded to the SWNT sidewalls. The TGA/MS (thermogravimetricanalysis/mass spectroscopy) for sidewall phenylated SWNT indicate thephenyl groups detach at about 400° C.

TABLE 1 Weight loss and estimated SWNT carbon/alkyl group ratio from TGAwith heating to 800° C. in argon. Weight loss Ratio of (wt %)^(a) SWNTExample Figure R group Observed by carbons to Number Number on SWNTsidewall TGA each R group None SWNT-pristine 3.6 NA 3 2 -Phenyl 31 14 44a —(CH₂)₁₇CH₃ 40 31 4 4b —(CH₂)₃CH₃ 42 6 4 4c —CH(CH₃)CH₂CH₃ 46 5 4 4d—CH₂CONH₂ 48 5 4 4e —(CH₂)₃Cl 44 8 4 4f —CH₂CN 30 8 4 4g—(CH₂)₃—O-THP^(b) 41 17 4 4h —CH₂COOCH₂CH₃ 35 13 4 4i—(CH₂CH₂O)_(m)(CH₂)₃CH₃ 26 45 where m~3 ^(a)All values were compensatedfor weight loss of about 2-3% due to degassing at low temperature.^(b)THP denotes “tetrahydropyran.”

Example 4

This example demonstrates further reactions of SWNT havingsidewall-functional groups. SWNT sidewall-derivatized with ethyl acetate(See 4h in Table 1) was made according to procedures given in Example 3.38.6 mg SWNT-sidewall derivatized with ethyl acetate was sonicated in 50ml 2M NaOH for 30 min. Alkaline hydrolysis was conducted at 100° C. for24 hours, followed by acidification with 2 M HCl at 100° C. for 24 hoursto convert the ethyl ester group to an acetic acid group. To furtherfunctionalize the nanotubes to an amide by reaction with an amine, thenanotubes sidewall derivatized with acetic acid groups were dried undervacuum and then sonicated in 1 ml of anhydrous DMF to form a dispersionin DMF. The dispersion was immediately added to 10 ml thionyl chloride(SOCl₂) and heated at 70° C. for 24 hours to convert the terminalcarboxylic acid group to a terminal acyl chloride. The single-wallnanotubes with sidewall-functionality having terminal acyl chloride werefiltered onto a 0.2 μm PTFE (polytetrafluoroethylene) membrane filterand rinsed with anhydrous THF (tetrahydrofuran) to remove excess SOCl₂.The resulting rinsed single-wall carbon nanotubes having sidewall groupsterminated with an acyl chloride were mixed with 1 g of octadecylamine(ODA or CH₃(CH₂)₁₇NH₂) (melting point, 50-52° C.) and heated at 90 to100° C. for 96 hours. The reaction of the acyl chloride with the ODAformed a SWNT having an alkyl amide sidegroup.

The reaction scheme for the preparation of SWNT with asidewall-functionalized amide from the SWNT having asidewall-functionalized alkyl ester is shown in FIG. 10. FIG. 10 shows areaction scheme wherein single-wall carbon nanotubes that have beensidewall functionalized with an ester are saponified with sodiumhydroxide and neutralized with hydrochloric acid to form single-wallcarbon nanotubes that have alkyl groups with terminal carboxylic acidfunctionality, which are further chlorinated with thionyl chloride toform an acyl chloride, which is further reacted with octadecyl amine toform an alkyl amide group that is bonded to the single-wall carbonnanotube sidewall. The subscript “x” indicates a variable number ofsidegroups on the nanotube. After the heating, the reaction mixture wascooled to room temperature and excess ODA was removed by collecting thesidewall-functionalized nanotubes on a filter and washing with copiousamounts of ethanol and chloroform. The single-wall carbon nanotubessidewall-functionalized with the alkyl amine were dried in the form of athick paper at 80° C. under vacuum for 12 hours. TGA weight loss was39.82%, indicative of a SWNT carbon-to alkyl amide group ratio of about39-to-1.

Example 5

This example demonstrates the sidewall functionalization of single-wallcarbon nanotubes with carbon centered radicals using the Minisci methodof making the carbon centered radicals. In the Minisci method, anorganic sulfoxide of the form R—S(O)—R′, where R, R′ can be an alkyl oraromatic group, reacts with a hydroxyl radical to generate .R and .R′radicals. Hydroxyl radicals can be generated in various ways, however,Fenton's reagent, a convenient means, generates hydroxyl radicals fromhydrogen peroxide using a divalent iron catalyst. The .R and .R′radicals covalently bond to the sidewalls of the carbon nanotubes. Forexample, methyl radicals can be generated from dimethyl sulfoxide viathe Minisci route and covalently bonded to the SWNT sidewall. Thereaction scheme is shown in FIG. 3.

Preparation of Sidewall-Alkylated-SWNT Using Alkyl Radicals Generated bythe Minisci Route

Single-wall carbon nanotubes were functionalized with alkyl groups usingvarious alkyl sulfoxides and the procedure that follows. The alkylsulfoxides used with this procedure include a) dimethyl sulfoxide, b)di-n-propyl sulfoxide, c) di-iso-propyl sulfoxide, d) di-n-butylsulfoxide and e) di-sec-butyl sulfoxide. Dimethyl sulfoxide, di-n-propylsulfoxide and di-n-butyl sulfoxide were obtained commercially fromAldrich. Di-iso-propyl sulfoxide and di-sec-butyl sulfoxide weresynthesized by oxidizing the corresponding di-alkyl-sulfides with sodiummetaperiodate (NaIO₄) according to the following reaction scheme:

R—S—R+NaIO₄→R—S(O)—R+NaIO₃

16 mg purified SWNT (1.3 mmol carbon) was sonicated for 30 min in 20 mlalkyl sulfoxide to form a SWNT/alkyl sulfoxide suspension. To thissuspension was added 2.891 g (10.4 mmol) FeSO₄.7H₂O, followed bydropwise addition of H₂O₂ (30%, 20.8 mmol) over a period of 30 min underargon at room temperature. The mixture was stirred for 2 hours, dilutedwith water, filtered over a PTFE (0.2 μm) membrane, and washedextensively with H₂O. The sidewall-alkyl functionalized nanotubescollected in the filter were then washed by suspending the nanotubes inwater, sonicating for 20 min, and refiltering. This washing procedurewas repeated once with water and twice with ethanol. The washedsidewall-alkyl-derivatized nanotubes collected in the filter were driedunder vacuum at 80° C. for 12 hours and looked like a black thin mat ofnanotubes resembling a buckypaper. The resulting sidewall-derivatizedSWNT were subjected to TGA analysis to determine the weight loss due tothe evolution of the particular R group and to estimate of the ratio ofcarbons associated with SWNT to the number of R groups bonded to theSWNT sidewall. The results are tabulated in Table 2 below.

Preparation of Sidewall-Phenylated-SWNT Using Diphenyl Sulfoxide byMinisci Route

16 mg purified SWNT (1.3 mmol of carbon) was sonicated in 50 ml of 1%SDS (sodium dodecyl sulfate) about 30 minutes to form an aqueous SWNTsuspension. The suspension was then filtered to produce a SWNT slurry.To the SWNT slurry was added 9.0 g (44 mmol) diphenyl sulfoxide andsonicated for 30 min to form a SWNT/diphenyl sulfoxide suspension. Tothis suspension was added 2.891 g (10.4 mmol) FeSO₄.7H₂O, followed bydropwise addition of H₂O₂ (30%, 20.8 mmol) over a period of about 30 minunder argon at room temperature. The solution was stirred, diluted withwater, filtered over a 0.2 μm polytetrafluoroethylene (PTFE) membrane,and washed extensively with H₂O and ethanol.

The resulting sidewall-phenylated SWNT was subjected to TGA analysis todetermine the weight loss due to the evolution phenyl groups andestimate of the ratio of carbons associated with SWNT to the number ofphenyl groups bonded to the SWNT sidewall. The results are given inTable 2 below.

TABLE 2 Weight loss and estimated SWNT carbon/alkyl group ratio from TGAwith heating to 800° C. in argon. Weight loss Ratio of (%)^(a) SWNTExample Figure R group Observed by carbons to Number Number on SWNTsidewall TGA each R group None None (pristine SWNT) 3.6 NA 5 5a —CH₃ 15 7 5 5b —CH₂CH₂CH₃ 18 16 5 5c —CH(CH₃)₂ 15 18 5 5d —CH₂CH₂CH₂CH₃ 17 22 55e —CH(CH₃)CH₂CH₃ 12 32 5 5f -Phenyl 13 40 ^(a)All values werecompensated for weight loss of about 2-3% due to degassing at lowtemperature.

Example 6

This example demonstrates the use of acyl peroxides for the preparationof sidewall-derivatized single-wall carbon nanotubes having sidegroupsterminated with carboxylic acid and amine terminated groups. Thecarbon-centered free radicals terminated with carboxylic acid groupswere generated from the decomposition of the diacyl peroxides havingterminal carboxylic acid groups. Examples of such peroxides include, butare not limited to, succinic acid peroxide and glutaric acid peroxide,derived from succinic anhydride and glutaric anhydride, respectively.The diamines used were ethylene diamine,4,4′-methylenebis(cyclohexylamine), and diethyltoluenediamine.

HIPCO single-wall carbon nanotubes were obtained, purified and preparedaccording to Example 1.

Preparation of Succinic (6a) and Glutaric Acid Peroxides (6b)

Succinic acid peroxide (6a) and glutaric acid peroxide (6b) weresynthesized using the one-step procedure of Clover, et al. Am. Chem. J.1904, 32, 43-68.10 g finely powdered succinic or glutaric anhydride(each obtained from Aldrich) were added to 20 ml ice cold 8% hydrogenperoxide and stirred for 30 minutes until all the powder dissolved and awhite gel-like solution formed. The solution was filtered through a 1-μmpore size PTFE (polytetrafluoroethylene) membrane (Cole Palmer). Thefiltrate was washed with small amount of water and air-dried for 10minutes. White peroxide product was collected from the membrane, putinto a glass vial and vacuum dried for 24 hours at room temperature.Approximately 6.5 g of each peroxide, 6a and 6b, were obtained usingthis procedure.

The peroxides 6a and 6b were analyzed by ATR-FTIR (Attenuated TotalReflectance-Fourier Transform Infrared Spectroscopy) using a ThermoNicolet Nexus 870 FTIR system equipped with ATR capability and solidstate ¹³C NMR (Nuclear Magnetic Resonance) to confirm the syntheses.ATR-FTIR spectra of both peroxides 6a and 6b showed similar features: abroad band in the 3000-3500 cm⁻¹ region due to the carboxylic O—Hstretches, peaks of the C—H stretchings in the 2850-3000 cm⁻¹ range,absorptions near 1700 cm⁻¹ characteristic of the acid carbonyl groups,and pairs of peaks at 1812 and 1779 cm⁻¹ assigned to the peroxidecarbonyls. CPMAS (Cross Polarized Magic Angle Spinning) spectra of acidperoxides 6a and 6b were obtained with a Bruker (50.3 MHz ¹³C, 200.1 MHz¹H) NMR spectrometer using 5 kHz MAS, a 1-ms contact time, 32.9-ms FID(free induction decay), and 5-s relaxation delay. The solid state ¹³CNMR spectra of 6a show three methylene carbon peaks at 29.9, 28.8, and24.7 ppm and three carbonyl carbon peaks at 181.7, 179.7, and 168.7ppm., and of 6b—three methylene peaks at 33.0, 28.4, and 19.0 ppm andtwo carbonyl peaks at 182.2 and 170.3 ppm.

Preparation of Sidewall-Functionalized Succinic Acid-SWNT (7a) andGlutaric Acid-SWNT (7b)

Sidewall-functionalized succinic acid-SWNT (7a) and glutaric acid-SWNT(7b) were prepared by the following procedure. 50 mg purifiedsingle-wall carbon nanotubes were placed into a 250-ml flask filled with50 ml dry o-dichlorobenzene and sonicated (17 W/55 kHz Cole Palmer bath)for 30 minutes to form a SWNT suspension. The SWNT suspension was heatedat 80-90° C. for 10 days while adding 0.5 g of peroxide 6a or 6b eachday. The carboxyl-alkyl radicals were thermally produced from thedicarboxylic acyl peroxides 6a and 6b (FIG. 6) which have half-lifes onthe order of one hour at 90° C. (according to Atofina Chemicals, Inc. asrelated at website www.atofinachemicals.com). Due to the stabilizinginductive effect of the carboxylic group, the reactivity of thecarboxyl-alkyl radicals with SWNT was somewhat reduced compared toundecyl and phenyl radicals. Therefore, a larger excess of peroxideprecursor over SWNT (˜10:1 weight ratio) was used to promote theaddition reaction (FIG. 7) by a concentration effect.

After the reaction was complete, the suspension was cooled and pouredinto a 500-ml Erlenmeyer flask containing a large amount oftetrahydrofuran and sonicated for 15 minutes to form a solution. Aftersonication, the solution was filtered using a 0.2 μm pore size PTFE(Cole Palmer) membrane. Functionalized SWNT (7a or 7b) that collected onthe membrane was placed into 100 ml of ethanol, sonicated for 20 minutesand the resulting suspension was refiltered. During filtration, ethanol,in copious amounts, was used to completely wash unreacted peroxides andreaction byproducts from the functionalized SWNT. After the ethanolwashing, the functionalized SWNT 7a or 7b was vacuum dried overnight at70° C.

Characterization of SWNT Derivatives by Optical Spectroscopy

Raman: Raman spectra of purified and functionalized SWNT were collectedwith a Renishaw 1000 micro-Raman system using a 780-nm laser source.

FIG. 11 shows a Raman spectra for pristine SWNT and certain embodimentsof the present invention, wherein the subscript “x” indicates a variablenumber of sidegroup attachments that is variable depending on thesidegroup and conditions of preparation:

-   -   1) pristine SWNT (FIG. 11A);    -   2) SWNT-(CH₂CH₂COOH)_(x) (FIG. 11B);    -   3) SWNT-(CH₂CH₂CH₂COOH)_(x) (FIG. 11C); and    -   4) SWNT-(CH₂CH₂COOH)_(x) after heating to 800° C. in argon per        TGA (FIG. 11D).

FIG. 12 shows FTIR spectra for certain embodiments of the presentinvention, wherein the subscript “x” indicates a variable number ofsidegroup attachments that is variable depending on the sidegroup andconditions of preparation:

-   -   1) SWNT-(CH₂CH₂COOH)_(x) (FIG. 12A);    -   2) SWNT-(CH₂CH₂CONHCH₂CH₂NH₂)_(x) (FIG. 12B);    -   3) SWNT-(CH₂CH₂CONHC₆H₁₀CH₂C₆H₁₀NH₂)_(x) (FIG. 12C); and    -   4) SWNT-(CH₂CH₂CONHC₆H(C₂H₅)₂CH₃NH₂)_(x) (FIG. 12D).

The Raman spectrum of purified SWNT, given in FIG. 11A, shows atangential mode shift at 1594 cm⁻¹ and radial breathing mode shifts at213, 230 and 265 cm⁻¹, indicative of SWNT nanotube diameters in therange of about 0.7 nm and about 1.2 nm.

Raman (FIGS. 11B and 11C) and IR (FIG. 12A) spectra of sidewallfunctionalized SWNT indicated the presence of aliphatic functionality.When organic groups were attached to the SWNT sidewall, such as in thecase of SWNT-(CH₂CH₂COOH)_(x) (7a), a prominent Raman peak appearing at˜1296 cm⁻¹ (see FIG. 11B) indicates a disruption in the aromaticπ-electron system of the sp² carbon hybridization (i.e., sp³hybridization is occurring at the points of reaction.). The breathingmode peaks were weaker in the sidewall functionalized spectrum versuspristine SWNT. The Raman spectrum of SWNT-(CH₂CH₂CH₂COOH)_(x) (7b), isshown in FIG. 11C, is similar to that of 7a (FIG. 11B) except that thesp³ mode peak at ˜1296 cm⁻¹ showed a slightly lower intensity which wasattributed to a somewhat lower number of organic groups attached to thenanotube sidewall surface for 7b compared to 7a.

No van Hove band structure was observed for sidewall functionalized SWNT7a and 7b by UV-Vis-NIR. The van Hove band structure is typicallyobserved with pristine SWNT. The sidewall functional groups were removedby heating sidewall functionalized

SWNT 7a and 7b in argon up to a temperature of 800° C. at 10° C./min ina TGA system. The defunctionalization of the sidewall derivatized SWNTwas evidenced by both weight loss and Raman spectra analyses. The Ramanspectra of the nanotube material collected after the pyrolysis showed adramatic reduction of the sp^(a) carbon mode intensity, (see FIG. 11D)which is indicative of defunctionalization.

Although IR spectra of pristine SWNT are featureless, ATR-FTIRspectroscopy of sidewall-functionalized SWNT showed features, such as inIR spectrum of the SWNT-derivative 7a shown in FIG. 12A. The peaks inthe 2800-3050 cm⁻¹ region indicate C—H stretches and a broad shoulderband in the 3100-3600 cm⁻¹ range indicate acidic O—H stretches. Thedominant peak at 1708 cm⁻¹ is attributed to the acid carbonyl stretchingmode. The broad band at 1384 cm⁻¹ is attributed to C—H bending and abroad peak at 1149 cm⁻¹ to the C—O stretching modes. The IR spectrum forsidewall-functionalized 7b was similar.

Example 7

This example demonstrates the preparation of amine terminatedsidewall-derivatized single-wall carbon nanotubes fromsidewall-functionalized, carboxylic acid-terminated SWNT made using acylperoxides.

Preparation of Sidewall-Functionalized Amide-SWNT 8

Sidewall-functionalized amide-SWNT 8 was prepared usingacid-functionalized SWNT 7a according to the reaction scheme diagrammedin FIG. 7. 20 mg acid-functionalized SWNT 7a was placed into a dry100-ml flask to which 20 ml thionyl chloride was added and the mixturestirred for 12 hours. The reaction mixture was then vacuum filteredthrough a 0.2 μm pore size membrane. A solid precipitate that collectedon the membrane was flushed with copious amounts of dry acetone and thendried in air. The air-dried precipitate was then placed into a 100-mlflask containing 20 ml ethylenediamine (Aldrich), and stirred for 12hours at room temperature. The stirred mixture was then poured intolarge amount of ethanol and sonicated for another 10 minutes. Thesonicated mixture was filtered through a 0.2 μm pore size membrane andthe precipitate flushed with copious amounts of ethanol. Theprecipitate, amide-functionalized SWNT 8, was collected on a membraneand dried overnight in a vacuum oven at 70° C.

Preparation of Sidewall-Functionalized Amide-SWNT 9

A similar procedure was applied for preparation of amide-functionalizedSWNT 9, according to the reaction scheme diagrammed in FIG. 8, by usingacid-functionalized SWNT 7b with SOCl₂ and4,4′-methylenebis(cyclohexylamine) (Aldrich).

Preparation of Sidewall-Amide-Functionalized SWNT 10

A similar procedure was applied for preparation of amide-functionalizedSWNT 10, according to the reaction scheme diagrammed in FIG. 9, by usingacid-functionalized SWNT 7b with SOCl₂ and diethyltoluenediamine(Aldrich). Unlike the reactions with ethylene diamine, given in FIG. 7,and 4,4′-ethylenebis(cyclohexylamine), given in FIG. 8, which proceededat room temperature, the synthesis of amide-functionalized SWNT 10 from7b required heating at 120° C. for 12 hours during the reaction withdiethyltoluenediamine.

Analysis of Sidewall-Amide-Functionalized SWNT 8, 9 and 10

IR: A Thermo Nicolet Nexus 870 FTIR system with ATR capability was usedto obtain ATR-FTIR spectra. UV-Vis-NIR spectra were obtained using aShimadzu 3101 PC UV/Vis/NIR spectrometer. The FTIR spectrum (FIG. 12B)of SWNT-(CH₂CH₂CONHCH₂CH₂NH₂)_(x) 8 (See FIG. 7) shows an intense peakat 3284 cm⁻¹ which was assigned to the N—H stretching mode of the amidemoiety. The carbonyl peak in 8 downshifts to 1691 cm⁻¹ with respect to7a as a result of the C(═O)NH linkage formation. The latter is alsocharacterized by the in-plane amide NH deformation mode observed at 1542cm⁻¹. The prominent peak at 1639 cm⁻¹ was assigned to the NH₂ scissoringmode in 8. The C—H stretchings in 8 are only slightly enhanced ascompared to 7a, the spectrum of which is given in FIG. 12A.

The SWNT-derivative 7b was readily converted into an amide derivative 9by SOCl₂ and 4,4′-methylenebis(cyclohexylamine) at room temperature inaccordance with the reaction in FIG. 8. In the IR spectrum of 9, shownon FIG. 12C, the features of the methylene C—H stretches at 2915 and2848 cm⁻¹ are greatly enhanced. The peak at 1711 cm⁻¹ and the broad bandat ˜3400 cm⁻¹ were assigned to the C═O and N—H stretching modes,respectively. The NH₂ scissor motion was associated with the peak at1626 cm⁻¹.

The amidation reaction, shown in FIG. 9, of diethyltoluenediamine with7b was negligible at room temperature, but proceeded upon heating at120° C. to yield the sidewall-wall functionalized SWNT 10, whose IRspectrum is shown on FIG. 12D. The bands at 2950 and 2867 cm⁻¹, as wellas those at 2916 and 2837 cm⁻¹ were assigned to CH stretches of the CH₃and CH₂ groups, respectively. The carbonyl stretching peaks at 1711 cm⁻¹and the NH stretching at ˜3400 cm⁻¹ were weak. The bands at 1621, 1451and 1375 cm⁻¹ were assigned to the NH₂ scissor, CH deformation andphenyl ring modes, respectively.

TGA/MS: Identification and quantification of thermal degradation andvolatile products were performed with a TGA/MS instrument equipped witha Q500 Thermal Gravimetric Analyzer coupled with a Pffeifer Thermal Starmass spectrometer. TGA-MS (Thermal Gravimetric Analysis-MassSpectroscopy) provided evidence for covalent attachment ofalkyl-carboxyl sidewall functionalized SWNT 7a and 7b. TGA was conductedover a range of 50-800° C. using 15 mg of 7a or 7b placed into a TGA panand heated at 10° C./min up to 800° C. under an argon flow. FIG. 13presents ion current versus time curves for molecular and fragment ionsoriginating from the evolution products detaching from the 7asidegroups. FIG. 13 shows TGA/MS of SWNT-(CH₂CH₂COOH)_(x), with peakevolutions due to the following sidegroup fragments and associatedatomic mass: a: —CH₂CH₃ (mass: 29); b: —COO— (mass: 44); c: —COOH (mass:45); d: —CH₂CH₂COOH (mass: 73); and e: —CH₂CH₂COO— (mass: 72). Theevolution curves were obtained for the parent ion (m/z 73) of thedetaching CH₂CH₂COOH radical, and fragment ions with the m/z 72{CH₂CH₂COO}, m/z 45 {COOH}, m/z 44 {COO}, and m/z 29 {CH₃CH₂}. That allcurves for the fragment ions have the same evolution temperatureindicates that the fragments originated from the same parent molecularion.

For 7a SWNT-(CH₂CH₂COOH)_(x), fragment evolutions started at about 170°C. which is comparable to that of methylated, butylated and hexylatedSWNT (˜160° C.) and relatively lower than phenylated SWNT (˜250° C.).Assuming that all weight loss was due to sidegroup detachment, thenumber of sidewall functional groups was estimated to be one functionalgroup for every 24 nanotube carbons for 7a.

Solid State NMR: Solid state MAS (Magic Angle Spinning) NMR spectra wereobtained with a Bruker (50.3 MHz ¹³C, 200.1 MHz ¹H) NMR spectrometer.MAS spectra were obtained by packing each sample in a 4-mm OD (outerdiameter) rotor and spinning at 7.5 kHz in the case of pristine SWNT andat 11.0 kHz for functionalized SWNT. The spinning sidebands appearedwell outside the sp² and sp³ centerband regions of interest (149 ppmupfield or downfield from the centerband in case of SWNT and 219 ppmupfield or downfield from a centerband for functionalized SWNT). Eachspectrum was obtained with a 4.5-μs 90° ¹³C pulse and a 32.9-ms,proton-decoupled FID, followed by a 180-s relaxation delay for pristineSWNT or a 45-s relaxation delay for sidewall-functionalized SWNT. Atotal of 1320 scans (66.0 hr) were averaged for pristine SWNT, 7152scans (89.5 hr) for the SWNT-derivative 7a, 4984 scans (62.3 hr) for 7b,and 3560 scans (44.5 hr) for the SWNT amide derivative 8. Each FID (freeinduction decay) was processed with 50 Hz (1 ppm) of line broadening.The resulting spectrum was phase corrected. A fourth-order polynomialwas then applied to the baseline over the region from δ 315 to −70 ppmto create a nearly flat baseline after the polynomial was subtractedfrom the spectrum.

A functionalized C₆₀ sample, C₆₀—(CH₂CH₂COOH)_(n), was used to estimatethe relative relaxation rates of the various types of carbons in thesample. For this estimate, a 7-mm OD rotor was filled withC₆₀—(CH₂CH₂COOH)_(n) and spun at 7.0 kHz. A spectrum was obtained with a3.8-μs 90° ¹³C pulse and a 32.9-ms, proton-decoupled FID, followed byeither a 15-s relaxation delay (10,440 scans, 43.6 hr) or a 45-srelaxation delay (2132 scans, 26.7 hr).

FIG. 14 shows ¹³C NMR spectra for pristine SWNT and certain embodimentsof the present invention, wherein the subscript “x” indicates a variablenumber of sidegroup attachments which is variable depending on thesidegroup and conditions of preparation:

-   -   1) pristine HIPCO SWNT (FIG. 14A);    -   2) SWNT-(CH₂CH₂CH₂COOH)_(x) (FIG. 14B);    -   3) SWNT-(CH₂CH₂COOH)_(x) (FIG. 14C); and    -   4) SWNT-(CH₂CH₂CONHCH₂CH₂NH₂)_(x) (FIG. 14D).

In FIG. 14A, ¹³C NMR showed a narrow isotropic peak at ˜δ123 ppm forpurified, pristine HIPCO SWNT. Sidewall functionalization with—(CH₂)_(n)COOH, n=2 or 3, (7a or 7b, respectively) resulted in abroadening and slightly upfield shift for the peak (FIGS. 14B and 14C).Functionalization also resulted in a carbonyl signal at about δ172 ppm,which, although broader than the CPMAS carboxyl signal of the precursorperoxide and exceptionally shielded for a carboxylic acid, wascorroborated by an IR absorption at 1708 cm⁻¹ consistent with acarboxylic acid. Although not meant to be held by theory, the π-systemof the nanotube itself may exert a shielding and broadening effect onthe carbonyl carbon of the substituent. For sidewall functionalizedSWNT-(CH₂CH₂COOH)_(x) 7a, a very weak broad signal consistent withaliphatic carbon is centered at about 820 ppm, while forSWNT-(CH₂CH₂CH₂COOH)_(x) 7b, signals from aliphatic carbons areessentially undetectable. For sidewall-functionalizedSWNT-(CH₂CH₂CONHCH₂CH₂NH₂)_(x), 8, the nanotube has little effect on themethylene signal. The amide group gives a signal at ˜δ174 ppm. Signalsfrom the methylene carbons are stronger than in theSWNT-(CH₂CH₂COOH)_(x) and SWNT-(CH₂CH₂CH₂COOH)_(x), 7a and 7b,respectively. The increased intensity is attributed to the relativelyremote, with respect to the nanotube, NHCH₂CH₂NH₂ methylene carbons,whose signals are somewhat upfield shifted.

All of the compositions and methods disclosed and claimed herein can bemade and executed without undue experimentation in light of the presentdisclosure. While the compositions and methods of this invention havebeen described in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to thecompositions and methods and in the steps or in the sequence of steps ofthe method described herein without departing from the concept, spiritand scope of the invention. More specifically, it will be apparent thatcertain agents which are chemically related may be substituted for theagents described herein while the same or similar results would beachieved. All such similar substitutes and modifications apparent tothose skilled in the art are deemed to be within the spirit, scope andconcept of the invention as defined by the appended claims.

1. A method for functionalizing carbon nanotubes, said methodcomprising: (a) providing hydroxyl radicals in the presence of anorganic sulfoxide to form carbon-centered free radicals; and (b)reacting the carbon-centered free radicals with carbon nanotubes to formfunctionalized carbon nanotubes.
 2. The method of claim 1, wherein thefunctionalized carbon nanotubes are sidewall-functionalized carbonnanotubes.
 3. The method of claim 1, wherein (a) the organic sulfoxidecomprises a first organic group R; (b) the carbon-centered free radicalscomprise first carbon-centered free radicals .R; and (c) thefunctionalized carbon nanotubes comprise the first organic group Rbonded to a sidewall of the carbon nanotubes.
 4. The method of claim 3,wherein the organic sulfoxide has a form R—S(O)—R.
 5. The method ofclaim 3, wherein the first organic group R comprises a number of carbonsin a range of 1 and about
 30. 6. The method of claim 3, wherein (a) theorganic sulfoxide further comprises a second organic group R′; (b) theorganic sulfoxide has a form R—S(O)—R′; (c) the carbon-centered radicalsfurther comprise second carbon-centered radicals .R′; and (d) thefunctionalized carbon nanotubes further comprise the second organicgroup R′ bonded to the sidewall of the carbon nanotubes.
 7. The methodof claim 6, wherein the first organic group R comprises a number ofcarbons in a range of 1 and about 30, and wherein the second organicgroup R′ comprises a number of carbons in a range of 1 and about
 30. 8.The method of claim 1, wherein the carbon nanotubes are selected fromthe group consisting of single-wall carbon nanotubes, multi-wall carbonnanotubes, sidewall-fluorinated carbon nanotubes, and combinationsthereof.
 9. The method of claim 1, wherein the organic sulfoxide is anaryl sulfoxide.
 10. The method of claim 9, wherein the aryl sulfoxide isdiphenyl sulfoxide.
 11. The method of claim 1, wherein the organicsulfoxide is an alkyl sulfoxide.
 12. The method of claim 11, wherein thealkyl sulfoxide is a dialkyl sulfoxide comprising a first alkyl groupand a second alkyl group; and wherein the first alkyl group and secondalkyl group are selected from the group consisting of a linear alkyl, abranched alkyl, a cyclic alkyl and combinations thereof.
 13. The methodof claim 12, wherein the first alkyl group has a number of carbons in arange of 1 and about 30 and the second alkyl group has a number ofcarbons in a range of 1 and about
 30. 14. The method of claim 1, whereinthe organic sulfoxide is selected from the group consisting ofdi-n-propyl sulfoxide, di-isopropyl sulfoxide, di-n-butyl sulfoxide,di-sec-butyl sulfoxide and combinations thereof.
 15. The method of claim1, wherein the hydroxyl radicals are generated using Fenton's reagent.16. The method of claim 1, wherein the hydroxyl radicals are generatedfrom hydrogen peroxide.
 17. A method for functionalizing carbonnanotubes, said method comprising: (a) providing methyl radicals in thepresence of an organic iodide, wherein the methyl radicals react withthe organic iodide to form carbon-centered free radicals; and (b)reacting the carbon-centered free radicals with carbon nanotubes to formfunctionalized carbon nanotubes.
 18. The method of claim 17, wherein thefunctionalized carbon nanotubes are sidewall-functionalized carbonnanotubes.
 19. The method of claim 17, wherein (a) the organic iodidecomprises an organic group R; (b) the carbon-centered free radicalscomprise .R carbon-centered free radicals; and (c) the functionalizedcarbon nanotubes comprise the organic group R bonded to a sidewall ofthe carbon nanotubes.
 20. The method of claim 17, wherein the carbonnanotubes are selected from the group consisting of single-wall carbonnanotubes, multi-wall carbon nanotubes, sidewall-fluorinated carbonnanotubes and combinations thereof.
 21. The method of claim 17, whereinthe methyl radicals are formed by a reaction of dimethyl sulfoxide andhydroxyl radicals.
 22. The method of claim 21, wherein the hydroxylradicals are generated using Fenton's reagent.
 23. The method of claim21, wherein the hydroxyl radicals are generated from hydrogen peroxide.24. The method of claim 17, wherein the organic iodide comprises anorganic group selected from the group consisting of an alkyl group, anaryl group, a cyclic group, and combinations thereof.
 25. The method ofclaim 17, wherein the organic iodide is an alkyl iodide.
 26. The methodof claim 25, wherein the alkyl iodide comprises an alkyl group selectedfrom the group consisting of a hydrocarbon alkyl group, an alkyl amide,an alkyl amine, alkyl halide, an alkyl cyanide, a nitro alkyl, an alkylether, an alkyl ester, an alkyl ether, an alkyl ketone, an alkylcarboxylic acid, an alkyl carboxylate and combinations thereof.
 27. Themethod of claim 25, wherein the alkyl iodide comprises an alkyl groupcomprising heteroatoms selected from the group consisting of nitrogen,oxygen, halogens, and combinations thereof.
 28. The method of claim 26,wherein the alkyl group has a number of carbons in a range of 1 andabout
 30. 29. The method of claim 27, wherein the alkyl group has anumber of carbons in a range of 1 and about
 30. 30. The method of claim19, wherein the organic group comprises a polymeric group.
 31. Themethod of claim 30, wherein the polymeric group comprises poly(ethyleneglycol).
 32. The method of claim 30, wherein the polymeric group isselected from the group consisting of a polyolefin, a polyester, apolyurethane, and combinations thereof.